The present invention relates to a method and apparatus which is used to retain spinal elements in spatial relationship. More particularly, a modular pedicle screw assembly of the invention consists of a base fastener and connecting headpieces which are varied in length and angularity in order to provide a clinician a greater degree of flexibility in stabilizing the spine when performing spinal,fusions in the cervical, thoracic, and lumbar regions.
Important considerations for spinal instrumentation used to promote spinal fusion are safe insertion, rigid fixation, and ease and adaptability of implantation. Historically, Harrington rods were an early type of spinal instrumentation used to stabilize the thoracic and the upper lumbar regions to obtain correction and stability for fusion. This instrumentation involved long rods attached to hooks, which were inserted over the bony arches (lamina) of the spinal canal. The instrumentation was subject to failure when the hooks either broke the lamina or became displaced and thus, the intended stabilization and correction was lost. Other rod systems have been developed which employ rod-screw connectors. One example is the TSRH(copyright) Spinal System available from Danek Medical Inc., 1800 Pyramid Place, Memphis, Tenn. 38132, (800) 93-DANEK. Even with current improved systems remains the challenge of attachment of the rods to the spine.
Other spinal instrumentation systems include the use of a longitudinal member, such as a plate, to interconnect a series of pedicle screws such as that disclosed in U.S. Pat. No. 5,129,899 issued to Small. In this system, the clinician must strive to place the series of pedicle screws into the bone in longitudinal alignment with as little lateral displacement and angular deflection as possible so that the plate may be attached to the series of pedicle screws in order to achieve a solid biomechanical relationship. However, due to variances in the spinal column or imperfect placement by the clinician, the screws are often inserted at an angle or must be placed laterally outside the longitudinally axis defined by the plate and other screws. While slight angular deviations of screw placement may be overcome by the use of contoured washers, larger angular deflections and the problems associated with lateral displacement of the pedicle screws cannot be overcome by the instrumentation disclosed in the Small patent. Moreover, the use of such contoured washers adds to the number of parts to be manipulated by the surgeon.
The development of the Kambin Offset Bolt (Smith and Nephews Richard, Inc.) for use with the device disclosed in the Small, et al. patent allows greater flexibility when the pedicle screws cannot be placed in the desired longitudinal relationship. The Kambin Bolt has a double threaded shank with an upper portion offset 6 mm from the axis of the lower portion. This offset bolt, however, has several drawbacks. First, insertion of the offset bolt requires a larger area of clearance to obtain insertion. This often leads to insertion difficulty when the bolt interferes with the spinal retractors, other elements of the spine, or other screws which have already been implanted. This interference can cause other implanted screws to loosen when impinged. In addition, once in place, the offset bolt can only overcome translational misalignment, but not angular misalignment of the pedicle screws.
U.S. Pat. No. 5,129,900 to Asher, et al. discloses a connector for pedicle screws and spinal rods. Lateral deviations may be cured by the use of connector members which include oblong openings. (Col. 4 lines 54-67). Like the offset screw taught by the Small patent, however, the device disclosed in the Asher patent cannot overcome the difficulties associated with angular misalignment.
Accordingly, a need has remained for devices which conveniently compensate for both translational and angular misalignment of bone fasteners relative to spinal devices such as rods and plates.
The present invention preserves the advantages of using a bone screw and longitudinal member while providing new features and advantages not found in the previous instrumentation. Specifically, the invention overcomes both translational misalignment and angular misalignment by providing a modular screw system having a fastener adapted to receive a plurality of headpieces with various angular and lateral length configurations which are, in turn, connected to a longitudinal member or members to obtain rigid stabilization of the spine.
In one embodiment of the present invention, a pedicle screw is provided having an exterior thread along its lower portion and an upper portion adapted to securely receive a headpiece. The headpiece includes a neck which has a bore adapted to mate with the upper portion of the fastener and further includes a projection which terminates in a connector which is adapted to engage a longitudinal member such as a plate or rod.
In another embodiment of the present invention, the upper portion of the pedicle screw is frusto-conical in shape with a fluted outer or xe2x80x9cmalexe2x80x9d surface; the headpiece including a corresponding inner or xe2x80x9cfemalexe2x80x9d portion. Mating the two components in this manner provides greater lateral and rotational stability.
In yet another embodiment of the present invention, an angulated headpiece is provided. The headpiece of this embodiment includes a neck portion having a projection which extends out of the neck at various angulations and terminates with a connector for engaging a longitudinal member. Angulating the projection allows for the correction of angular misalignment of the fastener.
In still another embodiment of the present invention, a translational headpiece is provided. The headpiece of this embodiment includes lateral sections which extend outwardly from the neck and may range in length from about 5 millimeters to 3 centimeters. The lateral sections also terminate in a connector for engaging a longitudinal member and allow for the correction of translational misalignment of the fastener.
In a further embodiment of the present invention, an angulated translational headpiece is provided. The headpiece of this embodiment includes angulated lateral sections which extend outwardly from the neck of the headpiece at various angles and lengths. These sections also terminate in a connector of both angular and translational misalignment of the fastener.
Not only does the present invention provide the clinician with the flexibility and adaptability needed to overcome difficult misalignment problems, it offers the patient a number of safety benefits as well. First, the present invention reduces the stress placed on both the placed fastener and patient""s bone by reducing the need to manipulate misaligned fasteners. Second, the integrity of the patient""s bone is maintained by eliminating the number of placements into the bone since misaligned fasteners do not need to be removed and/or relocated when the present invention is used by the clinician.
Accordingly, an object of the present invention is to provide a modular screw system and method of implanting the same which is more flexible, safer and easier to implant than present spinal instrumentation and methods.
Another objection of the present invention is to provide a modular screw system and method which can overcome translational misalignment of the fastener.
A further object of the present invention is to provide a modular screw system and method which can overcome angular misalignment of the fastener.
Yet another object of the present invention is to provide a modular screw system and method which can overcome both translational and angular misalignment of the fastener.
Still another object of the present invention is to provide a modular screw system and method which uses a fastener and headpiece having corresponding mating shapes in order to improve both the lateral and rotational stability between the two pieces.
Yet a further object of the present invention is to provide a modular screw system and method that can be used in combination with existing screw system employing either longitudinal plates or rods.